Antiribosomal P protein antibody in human systemic lupus erythematosus reacts specifically with activated T cells

We examined the presence of the epitope recognized by antiribosomal P protein antibody (anti-P) on the cell surface of human immunocompetent cells. Highly purified CD4+ and CD8+ T cells, and B cells from normal healthy individuals were reacted with affinity-purified IgG anti-P, and were stained with FITC-conjugated F(ab)2, fragment goat anti-human IgG, followed by analysis on flow cytometry with gating for viable cells by propidium iodide staining. The presence of an epitope that is antigenically related to the carboxyl-terminal 22-amino-acid sequence of ribosomal P protein was not demonstrated on the surface of fresh CD4+ and CD8+ T cells, or fresh B cells. However, the expression of the ribosomal P epitope was induced on CD4+ and CD8+ T cells after activation with immobilized anti-CD3, whereas the epitope was not expressed on activated B cells. These results indicate that anti-P is an antilymphocyte antibody, which reacts specifically with activated T cells but not with resting T cells or B cells, suggesting possible direct effects of anti-P on the immune dysregulation in systemic lupus erythematosus.